Osmosis

Understand the idea

Osmosis is the movement of water (or another solvent) through a partially permeable membrane from the side with more free water to the side with less free water. That is the same as moving toward the higher solute concentration. It is passive: the cell does not spend ATP on the water flow itself.

An isotonic solution matches the cell: water in equals water out. A hypotonic solution is more dilute outside, so water enters (endosmosis); the cell becomes turgid. A hypertonic solution is more concentrated outside, so water leaves (exosmosis); the cell becomes flaccid or plasmolysed. Osmotic pressure is the pressure that can stop that flow.

Animal cells survive in isotonic fluid; they can burst (lyse) in hypotonic liquid. Plant cells have walls, so hypotonic surroundings are useful: turgor supports the plant. In isotonic solution a plant may droop. Roots take water from soil by osmosis and send it up the xylem.

What to remember

• Osmosis: water through a membrane, high water → low water, no extra energy.
• Hypotonic: water in; hypertonic: water out; isotonic: no net change.
• Plants need turgor; animal cells lyse if too much water enters.

Recap

Osmosis is water following its concentration through a membrane. Link each solution name to the direction of water, then to turgor, drooping, or bursting.